1. Technical Field
The present invention relates generally to transmission system technology and, more particularly, to transmission system reclosing technology.
2. Description of the Related Art
Auto-reclosing is an economic and effective method that can improve the stability of a power system and the reliability of power supply.
In particular, since most faults occurring on a power transmission line are transient faults, it is important to rapidly restore a power system through a tripping operation and a reclosing operation.
Meanwhile, the result of auto-reclosing may be completely different depending on whether an occurring fault is a transient fault or a permanent fault.
That is, if auto-reclosing is performed when a transient fault has occurred, the stability of a system and the reliability of power supply can be improved. In contrast, auto-reclosing that is performed when a permanent fault has occurred may degrade the stability of the system and cause serious damage to expensive equipment inside the system.
Accordingly, although the discernment between a permanent fault and a transient fault is very important to the performance of auto-reclosing, research into this has not been sufficiently carried out.
Commonly used auto-reclosing methods include a single-phase auto-reclosing method and a three-phase auto-reclosing method.
The single-phase auto-reclosing method is a method that disconnects only a fault line when a single-line ground fault has occurred and recloses the line after a specific dead time. In the case of Korean domestic 345 kV transmission systems, most systems are designed to have a dead time of 36 cycles upon single-phase reclosing.
The three-phase auto-reclosing method is a method that performs three-phase tripping for all faults, except a single-line ground fault, in the case of, for example, a 345 kV transmission system or three-phase tripping for all faults regardless of the type of faults in the case of, for example, a 154 kV transmission system and then automatically performs reclosing after a predetermined fixed dead time. In the case of Korean domestic 345 kV transmission systems, most systems are designed to have a dead time of 48 cycles upon three-phase reclosing. 154 kV transmission systems are designed to have a dead time of 18 cycles upon three-phase reclosing.
Since these reclosing methods having a fixed dead time perform a reclosing operation when the state of a system is still unstable, they may exert an adverse influence on the system. Furthermore, a problem arises in that a dead time of tens of cycles must have elapsed even in the case where the state of the system has been stabilized immediately after tripping.
Accordingly, it is necessary to adaptively adjust the time at which a reclosing operation will be performed based on the stable state of a transient interval after tripping.
A conventional equal area criterion (EAC) method proposed as a transient stability assessment technique is suitable for a single machine infinite bus model, but has limited applications to actual transmission systems.
Meanwhile, integral square error (ISE) is used as an index related to the transient stability assessment. ISE may be used to assess the transient stability of a corresponding system after a reclosing operation has been performed. For example, the system may be viewed as being stable if the ISE converges to a specific value given in the corresponding system or converges to a rectilinear line having a specific slope, and may be viewed as being unstable if the ISE diverges.
However, since the reference of an ISE waveform that is used to determine whether a system in a tripped state is stable or unstable is ambiguous in the case where the ISE is used as an index for evaluating transient stability before a reclosing operation, it is not easy to determine the time at which a reclosing operation will be performed based on the ISE.